We have developed a compact and modular gamma camera system and assessed its performance when used on a small-animal SPECT prototype (rSPECT). Each camera consists of a Hamamatsu H-8500 position sensitive photomultiplier tube coupled to a 30 x 30 NaI (Tl) scintillator array (1.4mm x 1.4mm x 6mm crystal size) and electronics for pre-processing and matching the detector signals to an in-house developed data acquisition system. The camera components are enclosed in a lead-shielded case with a receptacle to insert the collimators (parallel-hole or pinhole with different tungsten apertures). System performance has been assessed for a low energy high resolution parallel-hole collimator (LEHR), and for a 0.75 mm pinhole collimator with 60° aperture angle. In this paper we present details on the system implementation and results of performance measurements, as well as first tomographic images on phantoms and animals. This SPECT was conceived for compactness and cost-effective routine small-animal imaging, and acquisitions of living mice and rats carried out with the system demonstrate its ability to provide useful high-resolution images for in vivo research. Index Terms - Single Photon Emission Computed Tomography (SPECT), small-animal imaging, gamma camera.